1. Field of the Invention
The present invention is related to radio-frequency (RF) structures, such as klystron or accelerator cavities wherein a special dielectric is used to support metallic elements inside the structure whereby superior electrical and thermomechanical performances are obtained.
2. Description of Prior Art
Present-day radio frequency (RF) cavities such as those in linear accelerators for charged particle beams typically comprise a series of metallic disks with a beam hole at the center. The metallic disks are separated by a distance equal or close to an integral fraction (1/N, where N is an integer) of the wave length of the pulsed RF power which energizes the beam. The RF frequency ranges widely from hundreds of megahertz to a tens of gigahertz and higher. In common practice, the inter-disk distance is maintained by a metallic spacer between two adjacent disks as in the case of a traveling wave (TW) structure, or by two or more metallic rods as in the case of a "plane wave transformer (PWT)" structure. A conventional PWT accelerating structure 10 is shown in FIG. 2. In this structure, the disks 12, having irises 13 formed therein, are connected by four rods 14 which also provide circulating coolant to the interior of the disks 12 to take away the heat generated by the coupling of the iris-loaded metallic disks to the RF field. The supporting rods 14 are connected to end flanges (not shown). The accelerating structure, or disk assembly, 10, is placed inside a cylindrical tank 16 which includes an input port for the incoming RF power. A unique feature of the PWT structure is that there is a very strong and efficient RF coupling between the electromagnetic field inside the outer tank 16 and all the accelerating cells between the disks 12. Another advantage of the PWT is that as a result of the strong coupling, the mechanical tolerance of the disks assembly is quite loose, thus making fabrication easier and less costly. On the other hand, there are several disadvantages of the PWT design shown in FIG. 2. The metallic rods 14 connecting the disks 12 distort the accelerating electromagnetic field, thus degrading the beam quality. The coupling of the magnetic field into metallic rods increases loss and reduces the shunt impedance and the quality factor of the accelerating cavity, thus lowering the efficiency. The method of heat removal in the metal disk by circulating coolant in its interior requires complicated design and expensive brazing of two halves of each disk. Finally, since the rods 14 are only supported at the two end flanges, excessive deflection at the center of a long disk assembly can only be avoided by increasing the diameter or wall thickness of the connecting rods, which would compound the electrical problems.